The present invention relates to a device for the generation of a blanking voltage for a cathode ray tube, and more particularly for generation of such a blanking voltage for a color television or display.
It is well-known that a television receiver comprises a cathode ray tube in which one or more electron guns are disposed. Electrons are beamed from the guns onto a screen composed of phosphors responsive to electron bombardment, and on which visual displays are thus created when the electron beam(s) scan(s) the screen. This scanning is effected along essentially parallel lines on the screen. The light intensity at each point scanned on the screen depends on the intensity of the electron beam; more precisely, if the intensity of the electron beam is equal to zero the display is black; when this intensity exceeds a certain level the display is white (in the case of black-and-white television).
The intensity of the electron beam generated by the gun of a television tube depends on the potential or voltage of the gun's various electrodes. In order to vary the intensity of the electron beam and, thus, the light intensity at each point scanned, it is generally the potential of only one electrode that is made to vary in the gun, most frequently that of the cathode, the potential of the other electrodes then remaining constant. The potential applied to this electrode presents a steady-value component on which is superimposed a component whose amplitude varies as a function of the intensity of the image to be reproduced. The variable potential is called the video voltage signal. The constant-value component is called the "blanking voltage" in the present description and makes it possible to produce a black display bordering on gray when the video signal is equal to zero. It should be stressed at this point that this constant-value component is a continuous component which must not be confused with the mean value of the video signal which is also frequently called "continuous component."
The blanking voltage must be equal to the extinction voltage of the tube and has a well-defined value: the precise value needed for a blank display. If the blanking voltage is not at that precise level, the display will be too light or too dark. For example, if the video signal is applied to the cathode and if the blanking voltage has a high positive value, the display will be permanently blank.
It is well-known that the value of the extinction voltage can be altered by several causes, in particular the aging of the television tube. In addition, the devices used for the generation of the blanking voltage can be subject to drift. If the blanking voltage is not modified in accordance with the changes in the extinction voltage and in accordance with said drift, the display will be affected, the latter then being, as stated above, either too bright or too dark; and in the case of color television it is the color that would be affected. That is why television receivers are generally equipped with devices that make it possible to vary the blanking voltage.
In black-and-white television receivers, this correction is the one called "brightness control" and it is performed manually, for example by turning the control knob of a potentiometer.
Color television receivers comprise three electron guns and in their case three blanking voltages must be generated. However, these blanking voltages can all vary by the same amount; this is a "common mode" variation and the correction for this variation can be performed manually by means of a "brightness control" knob, just as in the black-and-white television receivers. These variations may also be of a differential type, however. I.e., the variations may not be of identical value for the three guns. Such differential variation, in actual practice, cannot be corrected manually.